Since their discovery in the early 1990s, carbon nanotubes have been the subject of intense scientific research directed toward developing techniques for synthesizing high quality nanotube materials, and evaluating their physical and chemical properties. Carbon nanotubes are allotropes of carbon comprising one or more cylindrically configured graphene sheets. Carbon nanotubes typically have small diameters (≈1-10 nanometers) and large lengths (up to several microns), and therefore may exhibit very large aspect ratios (length to diameter ratio ≈103 to about 105). Nanotube materials are often classified on the basis of structure as either single walled carbon nanotubes (SWNTs) or multiwalled carbon nanotubes (MWNTs). Research over the past decade has demonstrated that carbon nanotube materials exhibit extraordinary mechanical, electrical and chemical properties, which has stimulated substantial interest in developing applied technologies exploiting these properties.
Single walled carbon nanotubes (SWNTs) are one class of carbon nanotubes which have been identified as potentially useful materials for a number of applied technologies. SWNTs are made up of a single, contiguous graphene sheet wrapped around and joined with itself to form a hollow, seamless tube having capped ends similar in structure to smaller fullerenes. SWNTs typically have very small diameters (≈1 nanometer) and are often present in curled and looped configurations. The energy band structure of SWNTs varies considerably, and SWNTs exhibit either metallic or semiconductor electrical behavior depending on their precise molecular structure and diameter. Under some experimental conditions, SWNTs undergo efficient self assembly processes that generate bundles (or ropes) of SWNTs aligned along their lengths and strongly bound together by van der Waals forces. SWNTs are chemically versatile materials which have been demonstrated as capable of functionalization of their exterior surfaces and capable of encapsulation of materials within their hollow cores, such as gases and molten materials.
Research over the last decade has identified a number of unique properties of SWNTs which make these materials particularly promising candidate materials for a variety of device applications ranging from a revolutionary class of new electronic devices to composite materials having enhanced mechanical properties. First, SWNTs are believed to have remarkable mechanical properties suggesting their utility as structural reinforcement additives in high strength, low weight and high performance composite materials. For example, calculations and experimental results suggest that the SWNTs have tensile strengths at least 100 times that of steel or any known other known fiber. In addition, SWNTs are stiffer than conventional reinforcement materials, such as carbon fibers, while also exhibiting a very large Young's Modulus (as large as about 1 TPa) when distorted in some directions. Second, SWNTs exhibit useful electrical properties which may serve the basis of a new class of nanotube based electronic devices. For example, the electron transport behavior in carbon nanotubes is predicted to be essentially that of a quantum wire, which has stimulated interest in fabricating ultrafast nanotube based devices. In addition, the electrical properties of SWNTs have been observed to vary significantly upon charge transfer doping and intercalation, which has opened up new avenues for tuning the electrical properties of these materials. Further, due to their nanometer size diameter, mechanical robustness, chemical stability and high electrical conductivity, SWNTs may provide enhanced field emitters in a range of devices, including flat panel displays, AFM tips and electron microscopes. Finally, SWNTs are also believed to possess useful thermal, magnetic and optical properties which make them suitable materials for a range of emerging applied technologies.
The unique chemical and physical characteristics of carbon nanotubes is often severely attenuated or entirely lost when these materials are present with substantial amounts of impurities. Therefore, the successful development of nanotube based technologies taking full advantage of their extraordinary properties depends critically on the availability of sources of substantially pure nanotube materials. Currently available methods for synthesizing SWNTs, however, do not directly result in substantially pure samples containing these materials. Rather, conventional synthesis processes, such as techniques utilizing arc discharge, laser ablation, and chemical vapor deposition, yield a complex reaction product comprising a mixture of SWNTS, carbonaceous impurities and noncarbonaceous impurities. The impurity component of the reaction product generated using many of these techniques is very significant and SWNTs often comprise less than half of the reaction product by weight. Carbonaceous impurities generated in conventional synthesis processes are present in both single layer and multilayer configurations and include amorphous carbon, graphene sheets, graphite, incomplete and complete fullerenes and multiwalled nanotubes. Noncarbonaceous impurities commonly present in SWNT containing samples include residual metal catalyst particles, such as particles comprising nickel, yttrium, iron, molybdenum, palladium, and cobalt, and catalyst support materials, such as ceramic materials.
As a result of associative intermolecular interactions, such as van der Waals interactions, impurities and SWNTs in samples prepared via conventional synthesis techniques are present in highly coupled physical states. For example, the outer surfaces of SWNTs and bundles of SWNTs are typically heavily coated with a variety of single and multilayer carbonaceous impurities. In addition, metal catalyst particles unavoidably generated in catalytic synthesis methods are often entirely or partially encapsulated in high stable multilayers comprising carbonaceous impurities. Associative intermolecular interactions involving these materials present a unique challenge for purifying and isolating SWNTs in samples prepared by conventional synthesis methods. For example, the carbon multilayers surrounding metal catalyst particles severely reduce the effectiveness of purification via dissolution of the particles in acids provided to the sample. In addition, associative interactions between carbonaceous impurities and SWNTs pose significant problems for positioning, aligning and/or integrating SWNTs into desired device configurations.
Significant research has been direct toward developing methods of isolating and purifying SWNTs, due to the inability of conventional synthesis methods to directly produce substantially pure samples of high quality carbon nanotubes. Effective purification methods are capable of removing the wide range of different impurities that exhibit markedly different chemical and physical properties. In addition, effective purification methods are capable of selectively removing a majority of these impurities without causing significant damage to or destruction of the SWNTs. Furthermore, effective purification methods are capable high throughout processing of significant quantities of SWNTs in a relatively small number of efficient purification steps, thereby avoiding unreasonably long processing times.
A number of different approaches have been pursued in recent years for purifying SWNT containing samples. First, several techniques have been developed that are based on chemically modifying impurities to enhance their removal, including selective oxidation processes, such as gas phase oxidation, catalytic oxidation and acid oxidation, and nitric acid and hydrogen peroxide reflux methods. Although chemical modification techniques have been demonstrated as capable of enhancing the purity of SWNT containing samples, these treatments invariably destroy a significant portion of the SWNTs present in the sample, and are not effective at removing metal particulate impurities enclosed in carbon multilayers. Second, purification methods have been pursued based on microfiltration and cross flow filtration techniques. These techniques require a relatively large number of repeated filtration and suspension processing steps, however, making the procedures relatively slow and inefficient. Finally, purification of SWNTs via size exclusion chromatography has also been demonstrated. However, this approach requires use of surfactants for suspension of the SWNTs in the sample undergoing chromatographic separation, which can result in residual surfactant in the purified sample that can deleteriously affect the chemical and physical properties of the purified nanotubes.
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It will be appreciated from the foregoing that there is currently a need in the art for improved methods for purifying carbon nanotube materials, particularly SWNTs, such that these materials can be effectively integrated into a range of applied technology settings. Specifically, methods are needed for purifying SWNT containing samples generated via conventional SWNT synthesis methods, such as samples comprising mixtures of SWNTS, a range of carbonaceous impurities and metal or metal oxide catalyst particles. In addition, purification methods are needed that provide high yields of high quality nanotube materials, thereby minimizing loss or damage to SWNTs in a sample undergoing purification. Further, low cost and versatile purification methods are needed that are compatible with high throughput processing of large amounts of SWNTs generated by the conventional synthesis techniques.